1. Field of the Invention
The present invention relates to a sheet conveyance device installed in an image forming apparatus like a copier, a printer, a facsimile machine, or a multifunction peripheral having plural such functions, and the image forming apparatus having the sheet conveyance device.
2. Description of the Related Art
A known image forming apparatus like a copier, a printer, a facsimile machine, or a multifunction peripheral having plural such functions has an inverting conveyance path, through which sheets having images formed thereon are conveyed with their front and rear sides inverted, so as to perform double-sided printing or collate the sheets.
In an image forming apparatus described in Patent Document 1, the front and rear sides of sheets are inverted in a supplying path, through which the sheets are conveyed from a sheet feeding cassette to an image forming part, so as to form images on both sides of the sheets. That is, the supplying path acts also as an inverting conveyance path. In conveying the sheets having images formed thereon with their front and rear sides inverted so as to collate the sheets, the sheets are first fed to a double-sided conveyance path (first conveyance path) provided right before the inverting conveyance path and then inverted. Thus, the sheets are capable of being directly ejected without being guided to the image forming part again.
Further, an image forming apparatus described in Patent Document 2 has plural inverting conveyance paths on the upstream side of an image forming part in a sheet conveyance direction.
Further, an image forming apparatus described in Patent Document 3 has a dedicated inverting conveyance path to guide sheets the front and rear sides of which are inverted to an image forming part.
Further, an image forming apparatus described in Patent Document 4 has a dedicated inverting conveyance path other than a double-sided conveyance path, and is configured to be capable of ejecting sheets the front and rear sides of which are inverted in the double-sided conveyance path or the inverting conveyance path without guiding them to an image forming part again.
Next, a discussion is made of an operation (i.e., inversion and ejection) in which sheets having images formed only on one of their surfaces are collated and ejected.
First, in the image forming apparatuses described in Patent Documents 2 and 3, sheets the front and rear sides of which are inverted are conveyed to the image forming part again. Accordingly, even when the sheets having images formed only on one of their surfaces are collated and ejected, the sheets must pass through the image forming part. Therefore, the image forming apparatuses have disadvantages in that the paths required to eject the sheets become long and productivity (first copy time and PPM (Pages Per Minute)) is decreased. Further, as the conveyance paths become long, there is a high likelihood of causing jamming. Moreover, conveyance rollers, guide plates, and the like provided on the upstream sides of the image forming parts are brought into contact with the image forming surfaces of the sheets. Therefore, the image forming apparatuses have another disadvantage in that the conveyance rollers, the guide plates, and the like are likely to be stained by toner and images are easily degraded.
In the image forming apparatus described in Patent Document 1, the sheets having images formed only on one of their surfaces are temporarily fed to the path, which acts not only as the conveyance path through which the sheets are returned to the image forming part again so as to perform double-sided printing but also as the supplying path through which the sheets from the sheet feeding cassette are supplied, and are then directly ejected with their front and rear sides inverted. Accordingly, the image forming apparatus described in Patent Document 1 does not have a dedicated inverting conveyance path like those provided in the image forming apparatuses described in Patent Documents 2 and 3.
However, in order to invert and eject the sheets having images formed thereon with the image forming apparatus described in Patent Document 1, conveyance-path switching units (switching claws) must be provided at two branching parts (i.e., a branching part at which a third conveyance path 24 branches off a second conveyance path 23 and a branching part at which a fourth conveyance path 25 branches off the third conveyance path 24 shown in FIG. 1 of Patent Document 1). For this reason, an actuator (e.g., a solenoid or a motor) that drives the two switching units is also required. Further, the sheets having images formed only on one of their surfaces are temporarily fed to the path, which acts also as the supplying path through which the sheets are supplied from the sheet feeding cassette, so as to be collated and ejected. Therefore, the image forming apparatus has a disadvantage in that conveyance rollers, a guide plate, and the like provided to feed the sheets from the sheet feeding cassette are brought into contact with the image forming surfaces of the sheets and likely to be stained.
The image forming apparatus described in Patent Document 4 is configured to be capable of ejecting the sheets the front and rear sides of which are inverted without guiding them to the image forming part again. Therefore, compared with the image forming apparatuses described in Patent Documents 2 and 3, the image forming apparatus can reduce the entire length of the conveyance paths and thus improve its productivity. Further, in collating and ejecting the sheets to which single-sided printing is applied, the image forming apparatus is free from a problem in which conveyance rollers, a guide plate, and the like provided on the upstream side of the image forming part are stained compared with the image forming apparatuses described in Patent Documents 1 through 3.
However, the image forming apparatus described in Patent Document 4 has the dedicated inverting conveyance path other than the inverting conveyance path acting also as the double-sided conveyance path. Therefore, it is necessary to secure space for the dedicated inverting conveyance path and separately provide conveyance rollers, a guide plate, and the like. This runs contrary to recent demand for downsizing image forming apparatuses and cost reduction.
Further, the image forming apparatuses described in Patent Documents 3 and 4 convey sheets through a switch back path only with inverting conveyance rollers. Therefore, when, e.g., long and thin sheets enter in a conveyance direction, the sheets are not properly guided to the switch back path and are buckled and damaged along the way of the path. Further, when the lengths of upper and lower conveyance guide plates, which restrict the length of the sheets in the conveyance direction accommodated in the switch back path, are insufficient, the tip end surfaces of the sheets act violently to get stuck on other members at the time of entering the switch back path, so that conveyance problems such as skewing and jamming are sometimes caused. In order to prevent this phenomenon, it is necessary to provide plural rollers along the way of the switch back path and secure sufficient lengths of the conveyance guide plates, which results in an increase in the number of components and cost. Further, even if the inverting conveyance path acting also as the double-sided conveyance path is used to invert the sheets, conveyance-path switching units (switching claws) must be provided at two parts like the image forming apparatus described in Patent Document 1 and thus an actuator (e.g., a solenoid or a motor) that drives the two parts must be required.
Further, in the image forming apparatus described in Patent Document 4, the image forming surfaces of the sheets are directed downward in the double-sided conveyance path so as to perform the switching back of the sheets on the double-sided conveyance path, while they are directed upward in the double-sided conveyance path so as to make the sheets enter the double-sided conveyance path via the dedicated inverting conveyance path. That is, the image forming surfaces are directed upward or downward in the double-sided conveyance path as required. Meanwhile, in the case of using, e.g., an ink jet method as an image forming method, rollers (point-contact rollers) having a small contact area with the image forming surfaces are employed as the conveyance rollers provided in the double-sided conveyance path so as not to influence undried image forming surfaces when the sheets enter the double-sided conveyance path with ink on the image forming surfaces being undried. However, when the image forming surfaces are directed upward and downward in the double-sided conveyance path as in the image forming apparatus described in Patent Document 4, the rollers (point-contact rollers) having a small contact area are required as the conveyance rollers on both of the upper and lower sides of the double-sided conveyance path. Therefore, this runs contrary to demand for cost reduction.
Patent Document 1: JP-A-2004-155553
Patent Document 2: JP-A-2006-213480
Patent Document 3: JP-A-2009-86506
Patent Document 4: JP-A-2009-40550